1. Field of the Invention
The present invention relates to the field of voltage regulators and more specifically to a method and apparatus for the control of a voltage utilized by a load, such as a DRAM, during periods where the load current fluctuates considerably.
2. Description of Related Art
Voltage regulator circuits are known in which a voltage supply to a load is regulated by regulating the current supplied to the load. One such voltage regulator is illustrated in U.S. Pat. No. 5,548,205.
Typical of such prior art structures is the use of a feedback circuit for sensing the output voltage which is used for comparison with a reference voltage with the difference between the output and reference voltages being used to control the current supplied to a load. With such circuits, when there is a considerable change in the current drawn by the load, the voltage regulator circuit also senses the large current drain and, compensates for it through the use of the negative feedback current sensing circuit to increase the current supplied to the load and thereby maintain the output voltage at a relatively constant level. Although such voltage regulators generally perform an adequate job of voltage regulation, a considerable amount of power and thus heat is drawn because of the use of the negative feedback circuit. In addition, the negative feedback circuit decreases the response time to sharp current fluctuations and also takes up considerable layout area when the voltage regulator is incorporated in an integrated circuit (IC) structure.
An adaptive voltage follower is also known which could be used as a voltage regulator and is shown in the text CMOS Circuit Design, Layout, and Simulation by Baker, R. J. et al at Chapter 26, FIG. 26.25, page 703. This circuit uses a differential amplifier to control an output voltage to a load based on changes to an inut voltage. The differential amplifier compares the output and input voltages and based on variations between the two generates a control signal which is used to control an output current control transistor to thereby control the output voltage. A feed forward current sensor formed by serially connected complementary transistors also receives the control signal and develops another control signal which partially controls the tail current to the differential amplifier. In this feed forward current sensing design, a current source is also required to ensure that an adequate tail current is always supplied to the differential amplifier. Although this curcuit could be adapted for use as a voltage regulator, and avoids the delay problem with a feedback current sensing approach, the differential amplifier used is unbalanced and a separate tail current source is required, making the circuit less accurate and more complex than desired. In addition, the output voltage is directly supplied to one input of the differential amplifier, so that output voltage connot be controlled to within desired limits, less than the limits of the supply voltage.
Additional problems also occur when a voltage regulator is used to regulate the supply voltage to a DRAM. In a DRAM an external voltage must be lowered and regulated during periods of considerable voltage and current fluctuation, for example, a DRAM load current may quickly fluctuate between microamps and milliamps during use. In order to accomodate such large current fluctuations a DRAM power suppy may use two separate power amplifiers for supplying operative power to the DRAM memory array, one of them a low power amplifier used to supply steady state current on the order of microamps, and another higher power amplifier for supplying transitory higher currents when needed on the order of milliamps. Typically the lower power amplifier supplies current during times of low current drain, while the higher power amplifier is switched on and operative only when needed during times of high current comsumption.
In addition, the higher power amplifier may in fact be constructed as a bank of lower power amplifiers, for example ten power amplifiers may be actually used, which are switched on in sequence as the required current to the load increases. That is, as more current is required additional amplifiers are turned on to meet the power demand. The control of multiamplifier power regulator circuits is complex requiring a control circuit for developing the necessary control signals for turning the various power amplifiers on and off on a dynamic basis in accordance with the required DRAM load current.
In addition, such multiamplifier voltage regulators tend to occupy considerable layout area when formed in an integrated circuit structure.